Is Pluto a planet? Showdown in Houston this week as scientists battle it out

The solar system may get a lot more crowded if one group of astronomers has it their way. Meeting this week at the Lunar and Planetary Science Conference in Houston, Texas, scientists from around the world are set to debate the fate of Pluto —planet or dwarf planet?— and with it determine how we look at some 110 other objects hurtling through outer space.

It all began in Prague in August 2006 when members of the International Astronomical Union determined the parameters for planet-hood and thereby swiftly kicked Pluto out of the club. Their threefold definition stated that planets must: (a) be in orbit around the Sun; (b) have sufficient mass to gravitationally pull itself into a round shape; and (c) have cleared its orbital path of all smaller bodies.

The rationale for the definition seemed legitimate enough. As space exploration expanded into the outer regions of the solar system, more and more so-called Trans-Neptunian Objects were being discovered, some which rivalled Pluto in size. In 2005, for example, Caltech astronomer Mike Brown discovered Eris, which is estimated to be 27 per cent more massive than Pluto, has its own moon and orbits around the Sun at a rate of once every 558 years.

If Pluto is a planet, the idea goes, Eris is, too, and so would be many more yet-to-be-discovered objects — hence the IAU’s definition and the subsequent demotion of Pluto.

But as it happens with any decision that breaks with tradition, controversy erupted, both from the lay public and the professional star-watching communities.

“This has been a can of worms really that’s been opened up and (there is) bad blood there between some scientists because when the voting took place back in 2006, many of the leading scientists weren’t even involved in the vote,” says Andrew Fazekas, the astronomy columnist with National Geographic, to Radio Canada International.

Runyon and colleagues are this week in Houston discussing the issue, critiquing the IAU’s planet definition and defending their own, one which has both simplicity and historical precedence on its side. “It’s a scientifically useful bit of nomenclature and, I think, given the psychological power behind the word ‘planet’, it’s also more consumable by the general public,” Runyon says to the Independent.

First, the problem with the IAU’s account is that one the one hand, it only recognizes objects around the Sun as planets, not objects around other stars, which, in the current age where so-called exoplanets are being discovered at a fast pace, makes no sense. On the other hand, part (c) of the definition requires zone clearing, something which, in reality, no planet does, since smaller bodies are constantly being injected into orbital paths, such as the Near Earth Objects.

Instead, the new definition states that planets are objects with sufficient self-gravitation to assume a round shape but are not so massive to undergo nuclear fusion (as the Sun and other stars do). The group states, “A simple paraphrase of our planet definition — especially suitable for elementary school students — could be, ‘round objects in space that are smaller than stars.’”

The one caveat? Scientists say there are 110 known objects that would qualify as planets under the new account. Try telling your Grade Four kid to memorize all of those.

About Jayson MacLean

Jayson is a writer, researcher and educator with a PhD in political philosophy from the University of Ottawa. His interests range from bioethics and innovations in the health sciences to governance, social justice and the history of ideas.